Golfers prefer golf clubs that exhibit performance characteristics such as forgiveness and playability. One measure of “forgiveness” can be defined as the ability of a golf club head to reduce the effects of mis-hits, e.g., hits resulting from striking the golf ball at a less than ideal impact location on the golf club head, on flight trajectory and shot distance. Greater forgiveness of the golf club head generally equates to a higher probability of hitting a straight golf shot. “Playability” can be defined as the ease with which a golfer can use the golf club head for producing accurate golf shots.
Golf club forgiveness is directly affected by the moments of inertia of the golf club head. A moment of inertia is a measure of the club head's resistance to twisting about the golf club head's center-of-center gravity, for example on impact with a golf ball. In general, a moment of inertia of a mass about a given axis is proportional to the square of the distance of the mass away from the axis. In other words, increasing the distance of a mass from a given axis results in an increased moment of inertia of the mass about that axis. Higher golf club head moments of inertia result in lower golf club head rotation on impact with a golf ball, particularly on off-center impacts with a golf ball, e.g., mis-hits. Lower rotation in response to a mis-hit results in a player's perception that the club head is forgiving. Moreover, higher moments of inertia typically result in greater ball speed on impact with the golf club head, which can translate to increased golf shot distance.